The parts of an exhaust system such as an exhaust manifold, an exhaust pipe, a converter case and a muffler are used in the environment of the exhaust system of an automobile. The parts are required to be excellent in a thermal fatigue property, a high-temperature fatigue property and oxidation resistance (hereinafter, these properties are collectively called a heat resistance). For applications in which the heat resistance is required as described above, nowadays, Cr containing steel to which Nb and Si are added such as Type429 (14Cr-0.9Si-0.4Nb) is often used. However, since an exhaust gas temperature has become higher than 900° C. in association with the improvement of engine performance, the thermal fatigue property of Type429 has become unsatisfactory.
In order to solve this problem, Cr containing steel having a high-temperature yield strength increased by adding Nb and Mo, SUS444 (19Cr-0.5Nb-2Mo) conforming to JIS G 4305 and a ferritic stainless steel containing less Cr to which Nb, Mo and W are added and the like have been developed (refer to, for example, Patent Literature 1). However, since the prices of rare metals such as Mo and W have been markedly rising recently, the development of a material having a heat resistance equivalent to these kinds of steel by using inexpensive raw materials has become to be required.
Examples of materials having an excellent heat resistance without using expensive chemical elements such as Mo and W are disclosed by Patent Literatures 2 through 4. Patent Literature 2 discloses a ferritic stainless steel to be used for the parts of an exhaust gas flow channel of an automobile in which Nb: 0.50 mass % or less, Cu: 0.8 mass % or more and 2.0 mass % or less and V: 0.03 mass % or more and 0.20 mass % or less are added to a steel having a Cr content of 10 mass % or more and 20 mass % or less. Patent Literature 3 discloses a ferritic stainless steel excellent in a thermal fatigue property in which Ti: 0.05 mass % or more and 0.30 mass % or less, Nb: 0.10 mass % or more and 0.60 mass % or less, Cu: 0.8 mass % or more and 2.0 mass % or less and B: 0.0005 mass % or more and 0.02 mass % or less are added to a steel having a Cr content of 10 mass % or more and 20 mass % or less. Patent Literature 4 discloses a ferritic stainless steel to be used for the parts of an exhaust gas flow channel of an automobile in which Cu: 1 mass % or more and 3 mass % or less is added to a steel having a Cr content of 15 mass % or more and 25 mass % or less. These kinds of disclosed steel are characterized by having a thermal fatigue property improved by adding Cu.